One-dimensional magnetism of Rh chains on the Ag„001… surface
D. I. Bazhanov,
1
W. Hergert,
2
V. S. Stepanyuk,
2
A. A. Katsnelson,
1
P. Rennert,
2
K. Kokko,
3
and C. Demangeat
4
1
Department of Physics, Moscow State University, Ru-117189 Moscow, Russia
2
Fachbereich Physik, Martin-Luther-Universita ¨t, Friedemann-Bach-Platz 6, D-06099 Halle, Germany
3
Department of Physics, University of Turku, FIN-20014 Turku, Finland
4
Institut de Physique et de Chimie des Mate ´riaux de Strasbourg, 23 rue du Loess, 67037 Strasbourg, France
Received 22 October 1999
The electronic and magnetic properties of one-dimensional Rh structures deposited on Ag001 surfaces are
investigated. The 010 and 110 chain directions with Rh atoms at ideal adatom positions are considered. A
semiempirical tight-binding method in a chain orbital representation is implemented to exploit the translational
symmetry along the chain directions. All structures considered display a sizable magnetic moment for the Rh
atoms in the chains. A nonconventional magnetic behavior due to interchain interactions is observed. The
results are in a good agreement with Korringa-Kohn-Rostoker calculations for small supported clusters.
I. INTRODUCTION
The electronic and magnetic properties of a low-
dimensional metallic surface system depend significantly on
the structure of the system. The enhancement of magnetic
moment in traditionally ferromagnetic materials and the
transfer of nonmagnetic materials into magnetic ones has
been obtained by reducing the size and dimensionality of the
considered systems
1
suggesting interesting magnetic materi-
als, e.g., for compact magnetic storage devices. In the past
mainly two-dimensional magnetic systems on single-crystal
surfaces have been studied.
1
However, modern experimental
tools e.g., diffusion controlled aggregation,
2
moving of at-
oms by the tip of a scanning tunneling microscope,
3
decora-
tion of steps
4
can be used to produce nanostructures of vari-
ous size and shape on different metallic substrates.
Very recently Gambardella et al.
5
have demonstrated, that
high density arrays of parallel nanowires of Ag and Cu can
be deposited on a vicinal Pt997 surface. Measurements of
Dallmeyer et al.
6
at Co nonowires on the vicinal Pt997
surface suggest the presence of a one-dimensional exchange-
split band and of local magnetic moments.
Transition metal TM structures on noble metal surfaces
attract special interest since in this case TM systems can be
considered as quasifree systems due to the relatively weak
interaction between transition metals and noble metals. First-
principles calculations for 3 d ,4 d , and 5 d transition metal
clusters on Cu001 and Ag001 substrates have shown cf.
Refs. 7 and 8, that transition-metal elements which are non-
magnetic in the bulk, could become magnetic due to the
increase of the lattice constants and the reduction of the di-
mensionality. Some of the theoretical predictions have been
confirmed experimentally.
9
The nonmagnetic 4 d element Rh has a special interest
due to its unexpected properties. Cox et al.
10
showed that
free standing Rh clusters are magnetic. Moreover, the aver-
age magnetic moment of free Rh clusters depends on the
symmetry and size of the clusters in a complicated way.
11,12
Therefore investigations on Rh systems could give better in-
sight on the interplay between the structure and the elec-
tronic and magnetic properties of low-dimensional metallic
sytems.
According to ab initio
13–17
and semiempirical
calculations
18
pseudomorphically grown Rh monolayers on
Ag001 could be ferromagnetically ordered with a magnetic
moment of 1
B
. However, experiments did not show any
magnetism for Rh monolayers. No magnetic moment was
found either with magneto-optical Kerr effect MOKE,
19,20
or with photoelectron spectroscopy.
21
This discrepancy is
suggested to be connected with the structural defects found
in real surfaces.
22,23
Detailed experimental investigations of
Rh surface layers have shown that the ideal layer structure
assumed in the calculations does not exist in real samples: it
is impossible to grow a Rh monolayer on Ag001 pseudo-
morphically without an additional Ag layer on top of the
structure. The direct interdiffusion process between Rh and
Ag atoms and the growth of mixed islands were observed at
Rh-Ag interface.
24
Thus experimental investigations suggest
that structural effects island formation, roughness at surface,
step defects, interdiffusion have strong influence on the
magnetic properties of low-dimensional metallic systems.
According to ab initio calculations by Freeman
17
and
Blu
¨
gel
25
the additional Ag layer on top of the Rh layer re-
duces the magnetic moment of Rh to 0.55
B
. Semiempirical
investigations
18,26
showed that an ordered layer Rh
50
Ag
50
on
the Ag001 surface destroys the magnetism of the Rh mono-
layer on the Ag001 surface. Later on Turek et al.
27
studied
the change of the magnetic properties in dependence on in-
termixture by means of the ab initio tight-binding TB- lin-
ear muffin-tin orbital LMTO- coherent potential approxi-
mation CPA method. The magnetic moments in the two
mixed layers of Rh
1 -x
Ag
x
/Rh
x
Ag
1 -x
/Ag(001) have been
found to be a minimum in the concentration range 0.5x
0.6. The moments are 0.3
B
in the topmost layer and
0.2
B
in the subsurface layer. In contrast with this theoreti-
cal analysis of magnetic properties in the monolayer regime
the moments of small supported Rh adislands are enhanced
due to the interdiffusion with Ag atoms.
28
Using weak local-
ization and anomalous Hall effect Beckmann and
Bergmann
30
have found an enhancement of the magnetic
moment of small Rh clusters compared to single Rh adatoms
PHYSICAL REVIEW B 1 SEPTEMBER 2000-II VOLUME 62, NUMBER 10
PRB 62 0163-1829/2000/6210/64156/$15.00 6415 ©2000 The American Physical Society